1. Field of the Invention
This invention relates to a steering structure of a two-wheeled vehicle such as a bicycle and a motorbike, and in particular to a head parts assembly forming a part of the frame of such a two-wheeled vehicle.
2. Description of the Prior Art
The frame of a bicycle includes a head parts assembly which rotatably supports a front wheel and which is connected to a handle bar through a handle bar stem thereby allowing to direct the front wheel in a desired direction with respect to the frame. Such a head parts assembly, typically, comprises a head pipe which is fixedly connected to the top and down pipes of a frame to be held generally vertically but slightly inclined toward a rider thereby defining a head angle between the head and top pipes. The top pipe is held to be substantially horizontal, and the down pipe extends downwardly at an angle from the head pipe to a bottom bracket of the frame. The head parts assembly also comprises a front fork which includes a front fork pipe, a front fork crown fixedly connected to the front fork pipe and a pair of front fork legs depending downwardly from the crown to rotatably support the axis of the front wheel at their tip ends. The front fork pipe is rotatably housed in the head pipe, and the bottom portion of a handle bar stem carrying a handle bar at its top end is fixedly fitted into the front fork pipe. Thus, the front wheel rotatably supported by the front fork may be directed in a desired direction by having the handle bar stem rotated around its longitudinal axis by the handle bar.
A prior art head parts assembly is typically constructed as shown in FIG. 1. The assembly of FIG. 1 includes a front fork pipe 1 which is provided with a threaded section 1a over a predetermined length from its top end. A bottom outer ring 3 is fixedly fitted onto the bottom end portion of the front fork pipe 1, which is integrally formed with a front fork crown 1b. A pair of front fork legs 1c, 1c extend downwardly from the front fork crown 1b, and although not shown specifically, it should be understood that a front wheel is rotatably supported by the pair of legs 1c, 1c at their bottom ends.
The head parts assembly of FIG. 1 also includes a head pipe 6 which is provided concentrically with and generally houses therein the front fork pipe 1. Although not shown specifically, it should be understood that the head pipe 6 is fixedly connected to top and down pipes in forming a frame. The head pipe 6 is provided with top and bottom inner rings 4, 5 at its top and bottom ends, respectively. A cavity is defined between the opposed curved surfaces of the bottom outer and inner rings 3, 5, and a ball bearing 9 comprised of a plurality of balls and a retainer is provided in the cavity thus defined.
A top outer ring 7 is threaded onto the threaded section 1a of the front fork pipe 1 so that the opposed curved surfaces of the top inner and outer rings 4, 7 define a cavity in which a ball bearing 8 is snugly fitted. With this structure, the head pipe 6 and the front fork pipe 1 are relatively rotatable from each other through the ball bearings 8 and 9, and, therefore, even if the head pipe 6 is stationarily held, for example, by having it integrally connected to top and down pipes of a frame, the front fork pipe 1, together with the crown 1b and the legs 1c, 1c, may be rotated relative to the stationary head pipe 6, for example, by manipulating a handle bar which is integrally connected to the front fork pipe 1 through a handle bar stem having its top end fixed to the handle bar and its bottom end tightly fitted into the front fork pipe 1.
The resistance to rotation of the front fork pipe 1 relative to the head pipe 6 may be adjusted by changing the position of the top outer ring 7 on the threaded section 1a. An intermediate nut 10 is screwed onto the threaded section 1a, and it is tightly screwed against the top outer ring 7 so that the ring 7 may be maintained at a set position through a double nut structure between the ring 7 and the intermediate nut 10. Also provided are a head lamp holder 11 for holding a head lamp at its forward end (not shown) and a cover nut 12 which is threaded onto the threaded section 1a and which has an inwardly extending flange portion at its top end.
In such a prior art head parts assembly, while the front fork pipe 1 is repetitively rotated back and forth relative to the head pipe 6, the top outer ring 7 may move from the set position thereby causing wandering of the rotating axis of the front fork pipe 1, which is disadvantageous in steering performance. Furthermore, any vibration imparted to the top outer ring 7 may also cause slackening of the whole assembly. Besides, a long time use causes the balls of the ball bearings 8 and 9 to be worn so that the assembly would be slackened also because of this reason. In such a prior art structure, whenever the assembly becomes slackened, the top outer ring 7 must be retightened. However, since such an adjustment requires experience and skills, the head parts assembly or an entire vehicle must be brought to a repair shop or the like where skilled people are. In addition, since a small gap is usually present between the top outer and inner rings 4, 7 in the above-described prior art structure, water such as rain drops and washing water may sneak into the assembly thereby rusting the inside structure.